It is proposed to investigate the mechanism and the sterochemistry of the oxidative steps in the transformation of androgens to estrogens. The stereochemistry of the introduction of the 19-hydroxyl into androst-4-ene-3, 17-dione will be determined with the use of (19R) and (19S) [1H, 2H, 3H; 14C]-androst-4-ene-3,17-dione. The chiral (19R)-and (19S)-androgens required for the study were synthesized in our laboratory and are available for these investigations. The origin of the hydroxyl group eliminated in the conversion of the C19-19-diol to C19-19-aldehyde will be defined with the use of [19-180; 14C]-C19-substrates. The proposed mechanisms for the conversion of the 19-oxoandrogen into estrogens will be evaluated with the use of [180-labelled intermediates.